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We measured the low temperature wavelength modulated absorption and low temperature photoluminescence spectra of relatively high purity (n~1014-1015 cm-3) 6H SiC boule and epi layers at high wavelength resolution (0.1-0.7 Å) to adequately separate and identify phonon-assisted absorption and recombination processes due to free excitons. As a result we have identified newly resolved or weak features in both spectra which we associate with previously unidentified momentum conserving (MC) phonons. We obtain the energies of 21 of the 36 possible MC phonons in 6H SiC and a more accurate estimate of the exciton bandgap, Egx = 3.0225±0.0003 eV. In several of the phonon-assisted absorption onsets we also observe fine structure and variations in the measured spin-orbit splitting, which relate to a fourfold splitting of the free exciton energy bands.

Owing to its hexagonal symmetry, indirect band gap, and relatively large unit cell, the electronic band structure of $4H$-SiC is comprised of a complicated series of anisotropic valence and conduction band extrema even very near to the uppermost valence band maximum and lowest conduction band minimum. This has presented a difficult challenge to those experiments which have attempted to resolve the small energy separations between these band extrema. To overcome this challenge, we have measured the wavelength-modulated absorption (WMA) spectrum of $4H$-SiC over a broader wavelength range (3500--3800 \AA{}) and at a higher resolution ($l0.1$ \AA{}) than in previous work. By comparing these measurements with the low-temperature photoluminescence spectrum in ultrapure $4H$-SiC, we have identified several features, which we attribute to a $56\ifmmode\pm\else\textpm\fi{}3$ meV crystal-field splitting of the valence band maximum or a $136\ifmmode\pm\else\textpm\fi{}3$ meV separation between the two lowest conduction band minima. We also show that the spin-orbit split-off valence band, which has been observed in previous measurements of $4H$-SiC, contributes to nonparabolic dispersion near the valence band maximum, and this is responsible for several previously misidentified features in the WMA spectrum. Finally, we report the first experimental measurement of fine structure splittings in the free exciton ground state, which manifests as four small ($0.7\ifmmode\pm\else\textpm\fi{}0.1$ meV) splittings in the WMA spectrum due to mass anisotropy and electron-hole exchange interaction.

We use thick, relatively high purity 4H SiC boule material to measure the wavelength modulated absorption spectrum with improved wavelength resolution and sensitivity with respect to previous work. We observe several small 0.6 ± 0.1 meV splittings, which we attribute to electron mass anisotropy and electron-hole exchange interaction. In addition, we identify several features in the absorption spectrum as signatures of nonparabolicity in the free exciton dispersion relations, the primary origin of which is likely the nonparabolic energy dispersion of the valence bands, as revealed by published band structure calculations based on density functional theory.

Differential absorption measurements were taken on ultra-pure boule pieces and epitaxial films of 4H SiC. The energy range of particular interest is from 3.40 eV to 3.52 eV. The free exciton energy gap associated with the second lowest conduction band at the M point in the Brillouin zone was determined to be EGX-2 = 3.4107 eV. This value is obtained from phonon assisted free exciton transitions involving the second conduction band measured in transmission with polarization E⊥c. The energy separation of the two lowest conduction bands is determined to be 144 ± 2 meV. Some replica peaks attributable to the spin orbit splitting in the valence band are also seen.

The formation of step bunches and/or facets on hydrogen-etched 6H-SiC(0 0 0 1) and ( 0 0 0 1 ¯ ) surfaces has been studied, using both nominally on-axis and intentionally miscut (i.e. vicinal) substrates. It is found that small miscuts on the (0 0 0 1) surface produce full unit-cell high steps, while half unit-cell high steps are observed on the ( 0 0 0 1 ¯ ) surface. The observed step normal direction is found to be 〈 1 1 ¯ 0 0 〉 for both surfaces. Hence, for intentionally miscut material, a miscut oriented towards this direction produces much better order in the step array compared to a miscut oriented towards a 〈 1 1 2 ¯ 0 〉 direction. For (0 0 0 1) vicinal surfaces that are miscut towards the 〈 1 1 ¯ 0 0 〉 direction, the formation of surface ripples is observed for 3° miscut and the development of small facets (nanofacets) is found for higher miscut angles. Much less faceting is observed on miscut ( 0 0 0 1 ¯ ) surfaces. Additionally, the (0 0 01) surface is found to have a much larger spatial anisotropy in step energies than the ( 0 0 0 1 ¯ ) surface.

Ultra-pure n-type (8×1013cm-3), 99 μm thick epitaxial films of 4H SiC were electron irradiated at 170 keV with a fluence of 5×1016cm-2or at 1 MeV with a fluence of 1×1015cm-2in various geometries. Low temperature photoluminescence (LTPL) spectra and microwave photoconductance (μPCD) lifetime measurements were obtained on all samples prior to annealing and after annealing in Argon in free standing mode or on a POCO carbon platform, every 50°C from 1100°C to 1500°C. No improvement in carrier lifetime was obtained. Spurious lines attributable to the use of a Genesis CX 3550Å laser are also reported.

Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.

The C-C stretch vibration associated with the dicarbon antisite in 4H SiC has been observed out to the fifth harmonic in the low temperature photoluminescence spectrum. The anharmonicity is accounted for reasonably well by fits to the data based on the Morse potential. We combine the observations from experiment, the analytically tractable Morse potential, and results obtained from first principles calculations on this defect to obtain an estimate of the thermal expansion coefficient of the C-C bond. This local thermal expansion coefficient is considerably smaller than the linear thermal expansion coefficient of bulk 4H SiC, in striking contrast with the recent result for the nitrogen-vacancy center in diamond that the local thermal expansion coefficient is larger than the bulk value.

Low doped epitaxial films of 4H SiC irradiated at 1 MeV and electron fluences of 1×1015 cm-2, 3×1015 cm-2 and 1×1016 cm-2 have been used to measure temperatures in two temperature intervals: 580°C to 640°C and 1220°C to 1320°C. Possible accuracy of the temperature measurements is judged to be better than 10 degrees Centigrade. Similar measurements should be possible from 100°C to 1500°C.

The analysis of the donor-acceptor pair luminescence of P-Al and N-Al pairs obtained recently for the cubic 3C polytype of SiC is viewed in some detail. A detailed consideration is given to the fitting procedure applied to the P-Al and N-Al spectra. Fit with theoretical models of spectra of type I and type II are applied to both N-Al and P-Al experimental spectra, and it is demonstrated that only contribution from P on Si site is observable in the presented samples. The accuracy of the obtained phosphorus ionization energy of 48.1 meV is also discussed.

In this paper we revisit sharp low temperature luminescence lines (LTPL) previously generated by high dose 1018 to 1020 cm-2 electron beams in an electron microscope and now produced by low dose 1015 cm-2 electron, 5x1010 cm-2 proton and helium ion irradiation. New no phonon lines E0, F0, θ0, Φ0, K0, G0, J0, M0 and phonon replicas are found. Phonon replicas up to the fifth harmonic are well accounted for by theory giving convincing new evidence that the di-carbon antisite is responsible for these deep defect lines.

We report measured and calculated frequencies of elastic waves propagating in columnar porous 4H-SiC, an analytically tractable system. The sample was prepared using photo-electrochemical etching followed by mechanical polishing. The frequencies were measured using Brillouin scattering spectroscopy in backscattering geometry. The effective elastic constants, ordinary and extraordinary indices of refraction, and mass density, all obtained using effective medium models, are used to calculate the frequencies. Although the quasistatic limit is not strongly satisfied, the agreement between the measured and calculated values is good.

Ultra-precision machining is dominated by single-crystal diamond cutting tools, and is typically applied to a narrow range of materials, particularly aluminum and copper. Single-crystal SiC can be comparable to some diamonds in hardness and thermal conductivity, while potentially having superior chemical and thermal stability, yet it has not been explored as a cutting tool for ultra-precision machining. We made two cutting tools with single-crystal SiC, one with sharp corners and one with a large circular radius, and used them to cut flat surfaces on two materials, 316 stainless steel and nickel. These materials generally cause unacceptably rapid diamond tool wear. We report the average roughness of the resulting surfaces cut with single-crystal 4H and 6H SiC tools.

This paper comprises a systematic study of the thermal stability of defect centers observed in n- and p-type 4H-SiC by deep level transient spectroscopy (DLTS); the defects are generated by irradiation with high-energy electrons of 170 keV or 1 MeV.

In this paper we describe an effort to find correlations between low temperature photoluminescence spectroscopy (LTPL) and deep level transient spectroscopy (DLTS) of electron irradiated samples annealed from 25 °C to 1700 °C in 100 °C steps. We report on thermal histories of defect centers created by 170 keV and 1 MeV electron irradiation, as observed by LTPL only. The DLTS results on "twin" samples are presented in a separate paper. Our results indicate that in n-type 4H SiC there is no correlation between the Z1/Z2 center in DLTS and the L1 peak of the DI center seen in LTPL. In p-type 4H SiC we do not find a correlation between a 350 meV DLTS peak above the valence band and the LTPL L1 peak of the DI center. Consequently, we cannot find evidence for a 350 meV ground state postulated in the “Pseudo–Donor” model [3].

Aluminum-doped 4H-SiC samples were either irradiated with high-energy electrons (170 keV or 1 MeV) or implanted with a box-shaped He+-profile. Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) were employed to search for defect centers. AS spectra of as-grown as well as of electron-irradiated (170 keV or 1 MeV) 4H-SiC epilayers reveal the Al acceptor (ΔE(Al) = 200 meV) and an unknown defect (ΔE(SB) = 177 meV), while AS spectra of the He+-implanted and annealed sample show in addition to the Al-acceptor two energetically deeper acceptor-like defect centers (ΔE(RE3) = 255 meV and ΔE(KR3) = 375 meV). The KR3-center is not directly formed by the He+-implantation, it requires an annealing process. The DLTS spectra of the He+-implanted and annealed sample resolve a double-peak structure of the KR3-defect (ΔE(KR3A) = 380 meV and ΔE(KR3B) = 410 meV).

High-temperature m -plane AlN nucleation layers have been used for the growth of planar GaN films by metalorganic chemical vapor deposition on (1 0 1¯ 0) m -plane 6H-SiC substrates. Structural studies using transmission electron microscopy reveal the presence of a novel AlN intermediary layer preceding the remainder of the 2H-AlN buffer layer. High-resolution observations and image simulations indicate that this initial AlN layer has a faulted hexagonal structure with a six-layer repeating stacking sequence of …CBCACBCBCACB… along the transverse [0 0 0 1] direction, which does not replicate the underlying 6H-SiC stacking. Based on image analysis, the space group of this novel phase is tentatively identified as P3m1, which is also hexagonal. A structural model of the m -plane AlN/SiC interface with periodic misfit defects is also proposed. Charge neutrality analysis indicates that the interface has an equal mixture of C–Al and Si–N bonds.

This article discusses the electrochemical polishing of silicon-face p-type 4H SiC using diluted aqueous HF solution. Etchings on the silicon and carbon faces of SiC samples are performed and compared. The experimental results show that the RMS surface roughness of the electrochemically polished Si-face could be as low as about 2 nm. Carbon-face electrochemical polishing gives a rougher surface. Therefore, silicon-face 4H SiC is a better candidate for MEMS processing. The underlying mechanism is also discussed.

The electronic and atomic structure of the 4H-SiC surface was investigated. Photoemission data indicate that the surface contains about 2 Si layers on top of the bulk layers. Scanning tunneling mic ...

The atomic and electronic structure of 4H-SiC(1 1 02) surfaces were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED) and photoemission (PES). Two well ordered phases existing on this surface, i.e. (2×1) and c(2×2) are discussed. The (2×1) phase consists of a Si adlayer which is topped by an array of ordered Si-nanowires with electronic states confined to one dimension. For the c(2×2) phase STM indicates the presence of adatoms and PES a surface composition close to bulk SiC stoichiometry. A detailed atomic model for this c(2×2) phase is proposed.

Planar m-plane GaN was grown on (1100) m-plane 6H-SiC substrates using high-temperature AlN nucleation layers by metalorganic chemical vapor deposition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed striated features on the sample surface aligned along the GaN [1120] direction, which are perpendicular to those associated with (1120) a-plane GaN. The epitaxial relationship between the m-GaN and 6H-SiC was analyzed using high-resolution x-ray diffraction (XRD). In order to reduce the defect density, epitaxial lateral overgrowth (ELO) was carried out on an m-GaN template with mask stripes along the GaN [1120] direction, which makes the lateral growth fronts advance along the GaN c-axis. On-axis XRD rocking curves show that the full width at half maximum (FWHM) values for the ELO samples were reduced by nearly half when compared to those of the m-plane template without ELO. Clear atomic steps were observed in the wing regions by AFM. The absence of the striated features that are associated with the template could be indicative of the reduction of basal stacking faults in the ELO wings. Low-temperature photoluminescence (PL) spectra showed an excitonic emission at 3.47eV, a basal stacking fault (BSF)-related emission at 3.41 eV, and other defect-related emissions at 3.29 eV and 3.34 eV.

The effect of the Schottky barrier height on the detection of the midgap defects EH6 and EH7 in 4H-SiC by deep level transient spectroscopy (DLTS) is systematically studied. The results show that the DLTS peak height - and as a consequence the observed defect concentration - increases with the increasing barrier height and saturates above 1.5 eV for EH6 and above 1.7 eV for EH7, while below 1.1 eV the DLTS peak height completely disappears. A model is applied, which determines the position of the quasi-Fermi level in the space charge region as a function of the barrier height and of the reverse bias applied and which explains the variation of the DLTS peak height.

Columnar porous Si-face 6H-SiC substrates were prepared by a photo-electrochemical etching method and applied as nanoimprint lithography (NIL) stamps. The diameter of the pores in the porous region was about 20 nm and the center-to-center separation between pores was about 60 nm. The columnar porous SiC substrates were subjected to a vapor phase silanization treatment whereby a monolayer of perfluorooctyltrichlorosilane (FOTS) was deposited in order to keep the stamps from sticking to the substrates during the imprint step. Subsequently, the porous SiC stamps were used to imprint polymethylmethacrylate (PMMA) at elevated temperatures and pressures. The imprinted PMMA could then be used to transfer the nanopattern on the columnar porous SiC to other substrates for various purposes; e.g. templates for GaN regrowth, catalysts for nanowire growth by vapor-liquid-solid type methods (VLS), etc. SiC is not typically used for NIL stamps since etch processing of SiC is less mature than that of Si. However, as demonstrated here, there is no reason why SiC cannot be used as a material for NIL stamps. The superior mechanical properties to Si make the use of SiC alluring as a master template for NIL processing.

The (1 over(1, ¯) 0 2) orientated plane of hexagonal silicon carbide of the 4H polytype consists of a periodic arrangement of stripes with alternating bond configuration on a nanometer scale. The two stripe configurations of the bulk truncated surface have an atomic structure very close to the carbon-face SiC basal plane and the cubic SiC(1 0 0) surface, respectively. The structural and electronic properties of the c(2 × 2) reconstruction on the 4 H-SiC (1 over(1, ¯) 0 2) surface were investigated using photoemission spectroscopy (PES), scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The core level photoemission spectra reveal two surface shifted Si2p components and one shifted C1s component in addition to the SiC bulk peaks. In accordance with the periodicity observed in LEED, atomically resolved STM micrographs show a c(2 × 2) arrangement of bright features which are accounted as Si adatoms. The electronic structure of this SiC (1 over(1, ¯) 0 2) -c (2 × 2) phase is experimentally determined by angle resolved PES studies of the valence band revealing four surface states. Based on the experimental observations and a comparison to similar phases on other SiC surfaces, a tentative surface model can be developed which consists of Si adatoms in so-called H3 sites on the basal-plane type stripes and carbon dimers in Si bridging configuration on the cubic stripes of the bulk truncated surface. © 2007 Elsevier B.V. All rights reserved.

Brillouin light scattering spectroscopy was used to probe porous silicon carbide films formed from p-type 6H crystalline silicon carbide. The porosities of the films ranged from 30% to 58%. Surface and bulk acoustic wave velocities were measured and compared with those calculated from the Mori-Tanaka acoustic effective medium model. Qualitative agreement is obtained between the experimentally determined velocities and those predicted by Mori-Tanaka acoustic effective medium models with spherical pores and, in the case of surface acoustic waves, also with prolate spheroidal pores with shape factor equal to 0.2. The model demonstrates the importance of morphology in determining the behavior of acoustic waves in a porous material.

A lightly doped n-type homo-epitaxial layer was grown by CVD onto a heavily doped n-type 4H-SiC substrate for which half of the surface had been made porous by photoelectrochemical etching. Raman spectra are obtained in the optic phonon region using three scattering geometries. An effective medium model for the porous layer is used to assist in the interpretation of the spectra. This work demonstrates that the contributions to the Raman spectra of the various layers in a sample with multiple 4H-SiC layers can be extracted.

We have fabricated columnar nano-porous SiC by photo-electrochemical etching on the C-face of n-type 6H SiC at constant voltage. SEM images reveal that the pores are long, straight and parallel with diameters of about 20 nm. We have produced such layers up to 250 μm thick. The pore morphologies for both Si and C-face SiC samples are compared and discussed as a part of the effort to understand the growth mechanism. It is found that the constant voltage etching condition on C-face SiC is crucial for this nano-columnar pore formation.

Ordered reconstruction phases on the 4H-SiC(1102) surface have been investigated using low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). After initial hydrogen etching, the samples were prepared by Si deposition and annealing in ultra-high vacuum (UHV). Two distinct reconstruction phases develop upon annealing, first with a (2×1), and at higher temperatures with a c(2×2) LEED pattern. After further annealing the fractional order LEED spots vanish and a (1x1) pattern develops. For the (2×1) phase, STM micrographs show that adatom chains develop on large flat terraces, which in view of AES consist of additional Si. These highly linear and equidistant chains represent a self-assembled well-ordered pattern of nanowires developing due to the intrinsic structure of the 4H-SiC(1102) surface. For the c(2×2) phase AES indicates a surface composition close to the bulk stoichiometry. For the (1×1) phase a further Si depletion is observed.

Brillouin spectra have been recorded for a series of supported films of p-type porous 6H-SiC with a branched morphology and porosities in the range from 30% to 58%. Complex spectra comprising up to 7 identifiable components were observed in some cases. An effective medium model is being developed as an aid in interpreting the spectra, and preliminary results are presented.

The effects of initial surface morphology on the early stages of porous SiC formation under highly biased photoelectrochemical etching conditions are discussed. We etched both Si-face and C-face polished n-type 6H SiC with different surface finishes prepared either by mechanical polishing or by chemical mechanical polishing at NOVASiC. For both Si-face and C-face porous SiC samples, a variety of surface and cross sectional porous morphologies, due to different surface finishes, are observed. The proposed explanation is based on the spatial distribution of holes at the interface of the SiC and electrolyte inside the semiconductor.

Optoelectronic devices with 1D modulation of the potential through hetero-structure or doping superlattices have so far been the privilege of III-V semiconductors. Based on the fact that SiC can be grown monolayer by monolayer, and that Si–Si and C–C double layers have been observed in it, we suggest the possibility of a stress-free polarization superlattice, consisting of the periodic variation of Si-face and C-face domains along the hexagonal axis of 4H-SiC. Such a structure could, in principle, be grown by molecular source atomic layer epitaxy. Investigating such superlattices by density functional theory, using a hybrid functional, we show that Si–Si and C–C double layers at the antiphase boundaries confine electrons within ~0.5 nm, and the periodic polarization field causes zig-zag shaped band edges which gives rise to tunable absorption, to spatial separation of free electrons and holes, as well as to optical nonlinearity. These properties could allow the application of SiC also in optoelectronics and photonics.

The high density of interface electron traps in the SiC/SiO2 system, near the conduction band of 4H-SiC, is often ascribed to graphitic carbon islands at the interface, although such clusters could not be detected by high resolution microscopy. We have calculated the electronic structure of a model interface containing a small graphite-like precipitate of 19 carbon atoms, with a diameter of ~7 Å, corresponding to the experimental detection limit. The analysis of the density of states shows only occupied states in the band gap of 4H-SiC near the valence band edge, while carbon related unoccupied states appear only well above the conduction band edge.

New lines are observed in the photoluminescence of 6H and 4H SiC epitaxial layers grown in cold wall CVD reactors and doped with phosphorus. These lines are associated with neutral phosphorus donor four particle bound exciton complexes with the phosphorus substituting on both the carbon and silicon sublattices. Assignments are made for the (h) hexagonal and (k) cubic sites of the phosphorus donor substituting on the two SiC sublattices.

Preliminary results for the phonon dispersion curves of hexagonal 4H-SiC from experimental inelastic neutron (INS) and X-ray scattering (IXS) are reported and contrasted with those of cubic 3C-SiC and silicon. The experimental frequencies and scattering intensities are in excellent agreement with those from first-principles calculations using density-functional methods. The relative merits of the two experimental techniques and aspects of the density functional perturbation theory and the so-called frozen phonon method for the determination of the basic phonon properties are briefly outlined.

A hybrid columnar and dendritic porous structure has been developed in n-type 6H SiC using photoelectrochemical etching with proper control of the applied voltage and current density. The diameter of the formed columnar pores is around 200-500 nm. A possible formation mechanism due to the spatial distribution of holes and the HF concentration gradient in the pores is proposed. A self-supporting film with this morphology is a promising candidate for protein dialysis.

The morphology and atomic structure of 4H-SiC(1102) and 4H-SiC(1102) surfaces, i.e. the surfaces found in the triangular channels of porous 4H-SiC, have been investigated using AFM, LEED and AES. After hydrogen etching the surfaces show steps parallel and perpendicular to the caxis, yet drastically different morphologies for the two isomorphic orientations. Both surfaces immediately display a sharp LEED pattern. Together with the presence of oxygen in the AES spectra this indicates the development of an ordered oxide. Both surfaces show an oxygen free, well ordered surface after Si deposition and annealing.

Hot-wall chemical vapor deposition has been used to epitaxially grow SiC layers on porous n-type 4H-SiC substrates. The growth was carried out at different speeds on porous layers of two different thicknesses. The quality of the SiC films was evaluated by X-ray diffraction and photoluminescence techniques. Based on the measurements, both the growth speed and the thickness of the porous layer buried underneath the epilayers do not appear to influence the structural integrity of the films. The intensity of the near bandedge low temperature photoluminescence appears stronger by a factor of two in films grown on porous layers.

We present relative recovery data for six proteins diffusing through porous silicon carbide membranes having a hybrid columnar/dendritic morphology. These membranes are promising candidates for implantable biosensors. The results are interpreted using an effective medium model.

Ab initio supercell calculations have been carried out to investigate the doping of phosphorus in chemical vapor deposited (CVD) SiC layers. CVD conditions have been simulated by using the appropriate chemical potentials for hydrogen and phosphorus (P). We found that the most abundant defect is P at Si-site followed by P at C-site. The calculated concentrations of the P-donors and free carriers in CVD grown SiC agree with the experimental results.

Three examples are given, which show that ion implantation and electron irradiation can drastically modify the electrical properties of SiC and SiC-based MOS capacitors. (1) It is demonstrated that sulphur ions (S^+) implanted into 6H-SiC act as double donors with ground states ranging from 310 to 635meV below the conduction bandedge. (2) Co-implantation of nitrogen (N^+) - and silicon (Si^+) - ions into 4H-SiC leads to a strong deactivation of N donors. Additional experiments with electron (e^-)-irradiated 4H-SiC samples (E(e^-)=200keV) support the idea that this deactivation is due to the formation of an electrically neutral (N"x-V"C","y)-complex. (3) Implantation of a surface-near Gaussian profile into n-type 4H-SiC followed by a standard oxidation process leads to a strong reduction of the density of interface traps D"i"t close to the conduction bandedge in n-type 4H-SiC/SiO"2 MOS capacitors.

A set of four lightly p-type 6H SiC boule samples was implanted with H or He and annealed in isochronal stages from 950°C to 1500°C. Differences in the hydrogen, DI and DII low temperature photoluminescence spectra are observed and compared. Surprisingly, the hydrogen spectrum appears after a 1300°C anneal in the He implanted samples. A number of unidentified damage lines are also reported.

Semi-insulating 6H SiC substrates, 2”, 3” and 100mm in diameter, and n+ 4H SiC substrates, 2” and 3” in diameter, are grown at II-VI using the Advanced Physical Vapor Transport (APVT) technique [1]. The process utilizes high-purity SiC source and employs special measures aimed at the reduction of background contamination. Semi-insulating properties are achieved by precise vanadium compensation, which yields substrates with stable and uniform electrical resistivity reaching ~ 1011 Ω-cm and higher. Conductive n+ 4H SiC crystals with the spatially uniform resistivity of 0.02 W-cm are grown using nitrogen doping. Crystal quality of the substrates, their electrical properties and low temperature photoluminescence are discussed.

The density of interface traps (Dit) in thermally oxidized SiC is unacceptably high for MOS device fabrication. The most severe problem is posed by the extremely high concentration of slow acceptor states near the conduction band edge of 4H-SiC. These states are attributed to near interface traps originating from (probably intrinsic) defects in the oxide. Here a systematic theoretical search is presented for possible defects in the oxide with an appropriate acceptor level. Supercell calculations using a hybrid functional approach (and resulting in a correct gap) on defects in alpha-quartz exclude the oxygen vacancy and the oxygen interstitial, as possible candidates. In contrast, these calculations predict interstitial silicon to have an acceptor level in the appropriate range. The carbon interstitial in silica has an acceptor level somewhat deeper than that. Occupation of these levels give rise to significant rearrangement of the environment, leading to a more extended defect structure.

The defects at the 3C-SiC/SiO2 interface have been studied by X-band EPR spectroscopy in oxidized porous 3C-SiC. One interface defect is detected; its spin Hamiltonian parameters, spin S=1/2, C3V symmetry, g//=2.00238 and g⊥=2.00317, central hyperfine interaction (CHF) with one carbon atom and AB//[001]=48G and superhyperfine (SHF) interaction with three equivalent Si neighbour atoms and TB//[001]=12.4G, allow us to attribute the center to a sp3 coordinated carbon dangling bond center, PbC.

A brief historical development of porous SiC and GaN is given. SEM images of nine porous morphologies in 4H, 6H and 3C SiC are shown along with anodization details. Similarly, two porous GaN morphologies are presented. Applications and future prospects are discussed.

Interaction of boron and aluminum with interstitial carbon is studied using first principles calculations. It is shown that carbon can form very stable complexes with Al and B, forming a family of negative-U bistable defects with deep levels. The influence of this effect on the activation rate of p-type implants is discussed.

Photoelectrochemical etching of highly doped n-type 4H SiC in dilute hydrofluoric acid along different crystallographic orientations under low voltage and/or low current conditions is studied. Scanning electron microscope images show that anodization of the hexagonal polytype 4H SiC with subsequent pore formation proceeds anisotropically. It is proposed that under uv illumination the crystallographic planes terminated with silicon atoms are more resistant to electrolytic attack than the planes terminated with carbon and mixed silicon-carbon atoms. This model is used to explain the observed triangular-channel pore morphologies. A clear indication was found that the resultant pore structure does not depend on the direction of the external electric field applied to the sample. Electrical parameters recorded as part of the photoelectrochemical etching process are described and interpreted.

This work demonstrates the creation of free-standing, nanoporous SiC membranes, and their ability to pass molecules of biologic interest. We have shown that proteins up to 29000 Daltons in molecular weight can pass through porous SiC. Also, we have shown that porous SiC resists protein adsorption comparable to the best commercially available polymer membrane that has been specifically developed to avoid protein adsorption.